The long-term goal of this project is to understand the development of auditory behavior during infancy. The the proposed work will determine how infants separate sounds from background noise and whether immature sound source separation contributes to intants'susceptibility to interference from competing sounds. Psychophysical methods and a rigorous behavior test procedure will be used to evaluate infants'perception, and to compare infants to adults. The initial studies proposed will use easily controlled sounds, tones and noises, to study sound source separation. In a final series of studies, these processes will be studies in infants'speech perception. The first specific aim is to describe the charactersitics of sounds that infants use to separate one sound from others. Infants and adults will be compared on their ability to detect a sound in the presence of sounds that vary randomly over time and in the presence of sounds that remain constant over time. A set of carefully characerized sounds will be used in a series of studies into the differences between sounds that allow them to be separated by infants and adults. The second specific aim is to determine how interference that takes place between sounds as they are coded in the ear and interference that arises in the central processing of sounds combine to interfere with the perceptual separation of sounds. Infants'and adults'ability to hear a target sound in the presence of interfering sounds will be exmined for interfering sounds that differ systematically in the types and amounts of interference they create. The third specific aim is to determine whether the principles of infant sound source separation identified in the initial proposed studies generalize to infants'separation of speech from competing sounds. Infants'ability to discrminate between vowels and between consonants will be assessed in the presence of constant sounds, sounds that vary over time and sounds that vary in their similarity to speech. The results of the proposed studies will show us how infants differ from adults in hearing and allow us to design appropriate programs for helping hearing-impaired infants to learn speech and language. These studies will also inform policy related to the acceptable levels of noise in places where infants and children live, play and learn.